Optical fiber cables are well known in the communication industry as cables that include one or more optical fibers for optically transmitting communication signals.
Among other constructions, one of the popular arrangements for optical fibers cables is a bundling of six to twelve individual optical fibers within a tube (also referred to as a buffer tube) in a loose arrangement, allowing for some movement of the optical fibers within the tube. This is referred to as a “loose tube” arrangement. Moreover, to form the optical fiber cable, one or more tubes may be bundled within an outer cable jacket for additional protection from the environment and also to provide an increased number of fibers within a particular cross section, useful for commercial installations.
However, there are several competing concerns that affect the design and production of such optical fiber cables. The first of these concerns is the optimum amount of fibers per tube. In typical installations larger optical fiber cables have multiple tubes therein. The greater the number of fibers per tube, the greater the overall communication capacity for the optical fiber cable. However, more fibers per tube may result in difficulty accessing individual fibers within a tube (e.g. for connection to optical equipment). Furthermore, more fibers add weight to the cable as well as geometrical constraints, both of which add costs in the form of materials and production difficulties.
A related second drawback to existing optical fiber cables of this design is the attenuation in fiber signals that occur when the optical fiber cable is bent. Attenuation occurs when individual fibers within an optical fiber cable are bent resulting in the optical signals being propagated therethrough to partially or totally exit the fiber. Increases in the number of fibers within each of the tubes in an optical fiber cable and their consequent geometric configuration, however restricts the possible movements of the fibers during bending, causing awkward and strained bending resulting in attenuation.
Prior art FIG. 1 shows an exemplary prior art arrangement optical fiber cable having seven fiber tubes within a jacket. Prior art FIG. 2 shows a hypothetical bend of the fiber cable from FIG. 1. The centrally located tubes (b) can conform to the center of the bent cable, but tubes along axes (a) and (c) are either stretched or compressed, resulting in signal attenuation. Thus, the more fibers placed in fiber optic cable the more attenuation in the fiber signal, particular with fibers closer to the inside wall of the cable jacket.
Given the constraints of attenuation from bending, combined with the desire to meet customer communication throughput needs by providing sufficient fibers per cable, prior art optical fiber cables are designed to include a substantial number of fibers per tube (typically between 6 and 12 fibers per tube). However, even with this range of fibers per tube, the attenuation at bend radiuses that occur in common installations results in significant signal attenuation.
To address this, prior art designs have added to the cable either strength members or binding ribbons to resist bending (or to prevent over-bending as some bending is required) or they have added fillers such as petroleum jelly or other gels, in either the tubes or around the tubes in the jacket. U.S. Pat. No. 4,230,395 discusses an example of such gel filled tubes. Yet another method of preventing attenuation in the fibers in these cables is to strand the fibers in a helical or S-Z arrangement so that no one fiber is consistently disposed along the far side of a bend axis.
All of these solutions are less than desirable. The addition of strength members adds additional construction components, adding cost in both materials and cable construction complexity. Furthermore, the strength members add additional weight to the final product. The addition of gel fillers also adds cost in both materials and extrusion complexity, adds weight, as well as the additional drawback of a fire fuel, which contributes to such gel filled cables failing the necessary fire safety standards for certain indoor uses.
Stranding, adds significant cost to the production of a cable in that the twisting of the fibers requires that more fiber per foot of cable is necessary to span a given distance relative to a straight or non-stranded fiber cable. Also, in the stranded arrangement, fibers acquire an inherent wavy quality that includes a certain amount of bending, which can result in failure of the cladding to contain the light signal through reflection, resulting in undesired attenuation.